The disclosure relates to methods and apparatus for extending the reliability and usefulness of a full width printhead by providing a redundant temporary replacement printhead module that can be positioned to compensate for missing or faulty jets.
Printers using full width printheads (i.e., printbars) are known and offer several advantages over conventional printheads that must travel back and forth across a print medium to achieve printing of a page. Advantages include faster printing speed, quieter operation, improved reliability due to less moving parts, etc. However, full width printheads suffer from certain drawbacks.
One particular drawback is a problem with defective nozzles. High productivity printers achieve enhanced productivity by employing a large number of nozzles. However, more nozzles result in greater opportunity for nozzle failure. For instance, a full width ink jet printhead array spanning a typical 8.5″ wide sheet of paper may have 7200 or more discrete individual jet nozzles, each of which must operate properly for the printer to produce a quality print. The problem is increased in high speed production architecture ink jet systems that can have a combined printhead width of up to 24″ or more.
Partially due to manufacturing limitations and partially to reduce the cost of replacement, many pagewidth or full width printheads use a number of smaller replaceable printhead modules rather than a large single head. The printhead modules are either butted together to form a single linear array, or offset and staggered in length to provide full width functionality. Such full width printheads may also include multiple printhead modules arranged in series (but offset by a partial pixel width to achieve an effective increase in resolution of the head itself).
For example, a prototype 24″ full color printer uses a first set of 32 modules (eight (8) three inch (3″) long 300 dpi staggered print modules for each of four colors C, Y, M, and K) to achieve 300 dpi printing. A second set of 32 printhead modules is offset by ½ pixel from the first set to effectively double the resolution of the printhead assembly to 600 dpi. Thus, 64 total printhead modules are present. This represents a total of 57,600 individual nozzles in the full width, full color printhead array. Having such a large number of individual jets increases the probability that any single ink jet will fail. This, coupled with very high printer usage in high speed production makes the probability and frequency of nozzle failure a significant problem.
A simplified example of this is shown in FIG. 1, which represents a single color printbar 100 having a first set 200 of printhead modules 200A-D and a second set 300 of printhead modules 300A-D. The first set 200 includes individual modules 200A-D that each contain a plurality of nozzles 210 spaced by a center-to-center distance S. The second set 300 similarly includes individual modules 300A-D that each contain a plurality of nozzles 310 spaced by a distance S. However, the nozzles in the second set 300 are offset from the nozzles in the first set 200 by a spacing S/2. This effectively creates a composite array with twice the resolution (i.e., an effective spacing of S/2) of the individual printhead modules.
In this simplified example, a defective nozzle 220 is present within printhead module 200B. As is evident from the vertical lines, nozzles from the offset printhead module 300B do not overlap with the single defective nozzle 220 shown. Accordingly, once at least one defective nozzle is present, the collective printbar 100 consisting of various printhead modules with nozzles is no longer capable of reproducing a complete image. Instead, the printbar 100 will print with a band or streak at the location of the defective nozzle where no printing can occur. Thus, once one or more nozzles become defective, image quality suffers.
Failed ink jet detection systems are known in the art. Such technologies include, for example, drop sensors that recognize missing or misdirected drops. One such drop sensing device uses a light beam that is projected across the width of the printing medium and between the printhead and the printing medium to a detector. Based on the timing and degree of occlusion caused by an ink droplet passing through the light beam, the device can sense the size and directional accuracy of the ink droplets. A laser may also be provided for such detection. Examples of suitable detectors include U.S. Pat. No. 5,179,418, the subject matter of which is hereby incorporated herein by reference in its entirety, as well as Japanese Patent Publication No. 4-315914 and Japanese Patent Publication No. 4-276446.
Even though nozzle failures, such as defective nozzle 220, can be detected, no practical method exists to repair individual failed printheads, other than minor problems that can be fixed through routine cleaning or maintenance. Rather, typical repair requires a complete replacement of the printhead module containing one or more defective print nozzles. This, however, is problematic for at least three reasons. First, the failed printhead module is typically thrown away, which represents a significant investment in cost, even though only a single nozzle or jet may be defective. Second, a replacement printhead may not be readily available, which can increase printer down time. Third, typical replacement and necessary alignment must be performed by a qualified technician, which requires additional printer down time to schedule and complete the replacement. Particularly when the printer involved is used for high volume production runs, there is a very high cost associated with the necessity to stop the current production run and make such necessary printhead repairs.
Various methods and attempts to improve the reliability of such printers are known, including for example, those disclosed in U.S. Pat. No. 5,581,284 to Hermanson, U.S. Pat. No. 6,089,693 to Drake et al., U.S. Pat. No. 6,462,764 to Kubelik, and U.S. Pat. No. 5,587,730 to Karz. Each of these four patents is commonly assigned to Xerox Corporation and hereby incorporated herein by reference in their entireties.